The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosures of all references cited herein are incorporated by reference.
Despite enormous progress in the ability to tailor the overall properties of nanoscopic particles, the ability to fabricate solid particle assembly structures targeting potential applications (for example, in optoelectronic devices, photovoltaics, and medical diagnostics) is limited because of difficulties associated with the processing of individual nanocrystalline materials into solid material assemblies over large length scales. These difficulties have severely limited scale-up of current assembly processes for technological applications.
The most common technique to form large-area two-dimensional (2D) or three-dimensional (3D) periodic superlattice structures, materials with an arrangement of particles with regular interparticle distance, is based on the self-assembly of ligand-coated particles in which the building blocks with controlled size and shape could/should spontaneously assemble into hierarchically organized structures through thermodynamic driving forces toward the equilibrium state. As used herein the term “superlattice” refers to a material that is compositionally modulated in a generally regular manner. Superlattices are periodic structures or layers of two (or more) materials. As used herein, the term “ligand”, refers to an organic material tethered to the surface of a particle that can alter the solubility or miscibility of the particle in a solvent or other materials. Superlattice structures include materials with an arrangement of particles with regular interparticle distance. However, the inherent nature of surface chemistry (for example, similarity in the chemical compositions and effective molecular volume) in the synthesis of nanoparticles imposes a constraint on the secondary intermolecular interactions. These constraints result in uncontrolled defect formation such as crack formation in the macroscopic particle structures. The ubiquitous existence of defects arising from self-assembly only utilizing weak cohesive forces (for example, van der Waals interactions) during the processing of the particle solids has resulted in limited progress in this field. Reports of procedures utilizing careful control of evaporative deposition conditions, or use of topologically patterned templates, each require extensive post-processing of particle films. See, for example, Small, 2009, 5, 1600-1630; Nat. Mater., 2008, 7, 527-538; Langmuir, 2009, 25, 6672-6677; Nano Lett., 2008, 8, 2485-2489.
The creation of large-scale particle film assemblies without non-equilibrium defects (such as crack formation during solvent evaporation that is driven by compressive stresses that arise during film shrinkage and domain disorientation) is necessary for high volume applications of particle assembly structures. However, previous studies report the fragile nature of particle assemblies to be a universal feature of such particle assemblies. There is thus a substantial demand to develop particle assemblies with improved cohesive interactions within particle assembly structures that do not require extensive post-processing of particle films.
The mechanical properties of films formed from solid particles including varying ligand systems has been attributed to a dominant role of dispersion interactions between the surface-bound ligands on the elastic properties of the particle array. See, Nat. Mater. 2007, 6, 656-660. Based on this attribution, the similar elastic characteristics of particle assemblies including different surfactant coatings may be explained as a consequence of the small interaction volumes, and the associated small molecular polarizability of the low molecular weight ligands that are bound to the particle surface. As a result of the small interaction volumes, and associated low levels of polarizability of low-molecular weight ligands, low values for both the elastic modulus (≦3 GPa) and hardness (≦0.1 GPa) have been observed for a wide range of surfactant systems such as oleic acid, trioctylphosphine, and dodecanethiol. The toughness of those particle solid structures was found to be of the order of 50 kPa m1/2, significantly lower than even brittle inorganic glasses.
As used herein ‘toughness’ describes the ability of a material to absorb energy during fracture. Low toughness values indicate more brittle fracture characteristics. Low toughness values are consistent with the widely observed susceptibility of particle solids to form cracks during fabrication. The development of techniques to increase the cohesive interactions within particle assembly structures without the need of extensive post-processing of particle films is thus an important prerequisite for broadening the range of applications for nanoparticle solids (for example, novel optical materials) that derive their properties from molecular interactions within particle superlattice structures.
Procedures to prepare hybrid particles including polymer grafts have, for example, been described in Chemistry of Materials 2001, 13, 3436-3448, WO 2002028912, Polymer Brushes 2004, 51-68, J. Am. Chem. Soc. 2003, 125, 5276-5277, J. Phys. Chem. B 2003, 107, 10017-10024, Annu. Rev. Nano Res. 2006, 1, 295-336 Advanced Materials 2007, 19, 4486-4490, Macromolecules 2009, 42, 2721-2728, J. Am. Chem. Soc. 2010, 132, 12537-12539, and Langmuir 2010, 26, 13210-5, the disclosures of which are incorporated herein by reference. Investigations have, for example, studied the effect of the architecture of polymer grafts on the static and dynamic properties of polymer-functionalized particle systems in good solvents. Exemplary polystyrene-coated silica particle brush model systems exhibited identical hard core diameter but distinct polymer-shell architectures corresponding to a concentrated polymer brush (CPB) regime and a semi-dilute polymer brush (SCPB) regime. FIG. 1A, for example, illustrates the CPB and SDPB regimes in the shell of a densely grafted particle brush system with a particle radius R0 and grafting density ρs. Within the terminology of a theoretical model that has often been used in the literature to analyze the structure of polymer-grafted particle systems, the CPB/SDPB transition occurs (as the molecular weight of the tethered chain increases) at the critical radius Rc, which depends on the grafting density, the molecular weight of the grafted chain and the radius of the particle. In the CPB regime, excluded volume interactions give rise to more stretched chain conformations. In the SDPB regime, the chains can begin to enter a more relaxed chain conformation. Although hybrid particles, including inorganic particles with surface tethered polymer grafts, show promise for providing relatively tough particle assemblies, improvements in particle order and mechanical properties thereof, and particularly a combination of relatively high order, improved mechanical properties and facile processability, remain very desirable.